Kaolin clays are hydrous aluminum silicates which, when purified, are white in color. These clays exhibit good covering or hiding power when used as a pigment or extender in coating and filling operations. Naturally occurring kaolin clay, however, contains discoloring contaminants such as iron and titanium. For certain industrial processes, such as coatings for premium grade papers, the level of these contaminants must be reduced in order to obtain a satisfactory coating product--a high-brightness clay.
In general industrial practice, iron contaminants can be removed by leaching methods using iron-reducing chemicals such as zinc or sodium hydrosulfite. However, no economical leaching chemical is known for discoloring titanium impurities.
There are three commercially employed practices for removing discoloring titanium impurities: high intensity magnetic separation, flotation, and selective flocculation. Unfortunately, high intensity magnetic separation is not effective for submicron particles and, therefore, it is difficult to remove enough discolored titanium impurities to produce high brightness clay. Though flotation and selective flocculation methods can be effective in producing high brightness clay, the currently available processes are either complicated and critical in processing, or result in undesirable residual chemicals after processing.
The objective of the present invention is to provide a novel, simple, and effective process for removing the discolored titanium impurities by selectively flocculating the impurities from kaolin clay. A further objective of the invention is to improve the brightness of kaolin clay with the minimum amount of processing chemicals and to produce a product which contains a minimum amount of undesirable residual chemicals. Yet another objective is to obtain high-brightness kaolin clay, a product having a G.E. brightness level of 89 or above. (In accordance with the G.E. brightness standard, pure MgO is assigned a G.E. value of 100. A number of 83.5 represents 83.5% of the MgO standard brightness.)
Two approaches are available for clay beneficiation by selective flocculation
1. The clay is flocculated and the impurities are left in suspension
2. The impurities are flocculated and the clay is left in suspension.
Generally, the first approach is not practical due both to the high consumption of reagents in flocculating the major constituent of the crude clay (clay particles) and to the resultant contamination of the clay product by the presence of the flocculating reagent. Examples of this first approach are presented in U.S. Pat. Nos. 3,477,809, 3,808,021, and 3,837,482.
The second method selectively flocculating the impurities, is a more practical approach to the clay beneficiation process. Because impurities are minor constituents in the composition, less reagent is used in the floccing step. This results in less entrapment of non-flocced matter, and thus product recovery is improved. Because a relatively low level of reagent is used, there is less reagent contamination of the clay product. Examples of this second approach are illustrated in U.S. Pat. Nos. 3,701,417 and 3,857,781.
In the procedure of U.S. Pat. No. 3,701,417, the impure clay is first deflocculated and the impurities liberated. The deflocculated slip is then pH adjusted and treated with polyvalent cations (e.g., calcium, magnesium, barium, zinc, etc.) Finally, an anionic polymer is added to the treated slip to flocculate the impurities.
It is well recognized in the clay processing field that the presence of polyvalent cations drastically increases the viscosity of the clay slip. As a result, the flocculation step of U.S. Pat. No. 3,701,417 must be operated at a low solids level--around 5 percent. Moreover, the residual polyvalent cations in clays produced by this process increase the product viscosity. A procedure which can be run at higher solids levels and which avoids residual polyvalent cations would be commercially advantageous.
U.S. Pat. No. 3,857,781 shows a multi-step flocculation procedure for removing titanium impurities from clay. The preferred salt utilized as a conditioning agent in this process is sodium chloride, although the patent states that other salts, including ammonium chloride, can be employed. The U.S. Pat. No. 3,857,781 teaches that satisfactory purification of a clay dispersion can be achieved if the clay is flocced twice. As a result, a large amount of dispersant must be utilized in the process to reflocculate the clay. This reflocculated clay is further treated with a high dosage of sodium chloride and requires substantial aging before the addition of the polymer flocculant.
The process of U.S. Pat. No. 3,857,781 is effective in impurity removal and can be operated at an economical flocculation solids of around 20 percent. However, the level of dispersants and conditioning agents in the treated clay make multi-washes of the product unavoidable and the presence of excess dispersant requires a high dosage of acid and aluminum sulfate for product coagulation before filtration or washing. (Generally, nine pounds of sulfuric acid and fourteen pounds of aluminum sulfate per ton of clay are required as described in examples of the patent).